Combustion engines emit an exhaust gas flow containing different kinds of pollutants into the atmosphere. Various apparatus for exhaust gas post-treatment have been developed to prevent the emission of pollutants. For example, catalysts are used for the conversion of harmful gaseous substances into harmless components and particulate filters are used for the capturing of unwanted solid particles. The exhaust tract of a diesel engine can, for example, be provided with an oxidation catalyst and a particulate filter arranged downstream thereof. Sooty particles, which are located in the exhaust gas flow are captured by the particulate filter and are stored in it. From a specific quantity onward, the collected soot has to be removed from the particulate filter so that the emission of exhaust gas is not prevented in too unacceptable a manner. This procedure is called regeneration.
The regeneration of the particulate filter in particular takes place by burning the soot, for which purpose a minimum temperature of the particulate filter is required. The exhaust gas temperature is, however, not sufficiently high in all operating states of the combustion engine. It is therefore also known to provide a burner in the exhaust tract by which the exhaust gas temperature can be raised to burn the soot in the particulate filter. It is also known to inject fuel into the exhaust gas which reacts exothermically in the oxidation catalyst and thereby heats the exhaust gas. However, a minimum temperature of the exhaust gas is also required for this since no exothermic reaction of the fuel in the oxidation catalyst takes place below a so-called light-off temperature.
Furthermore, nitrous oxide catalysts are used to reduce the nitrous oxides contained in the exhaust gas to nitrogen and water in a so-called selective catalytic reaction. These catalysts are therefore called SCR catalysts. In this respect, it is necessary to inject a reductant into the exhaust gas to achieve the selective catalytic reduction. A water/urea mixture is in particular used for this purpose whose urea decomposes in the exhaust gas to ammonia which reacts with the nitric oxides.